FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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CASE 3 ABUTMENT IS SET BACK FRIJII MIN CHANNElaE llWI 2.75 y.

There is overbank flow with no bed material transport (clearwater scour).
Figure 4.8 illustrates this case.

Main Bridge

SetBock~.
1>2.75y, r------'-'

R.li.f
Bridge

y. · Avg. Approach
Depth

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FIGURE 4.8 Bridge Abutment Set blck from Main Channel Bank
Ind Relief Bridge

With no bed material transport in overblnk flow, scour It I bridge abutment,
set blck more thin 2.75 times the scour depth from the ..in chlnnel bank line,
can be calculated using equation 8 from Llursen (1980) with:

t -
o -

Shear stress on the overbank Irea upstream of the
abutment.
Critical shear stress of .ateria1 in overblnk arel.
Can be determined from Figure 4.5.

t -
c -

Note 1. Values of the critical shear stress tc can be determined from figure
4.5 using the 0.0 of the bed material of the cross.section under
consideration. Alternately, they cln be calculated using the Shield's
relation for beginning of motion given in Highways in the River
Environment by Richardson et 11 (6).

Note 2. When there are relief bridges the a in equation 8 is taken as 1m.

Note 3. The lateral extent of the scour hole is nearly always determinable
from the depth of scour and the natural angle of repose of the bed
material. Llursen suggested that the width of the scour hole is
2.75y..

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